The Thousand-Year Door is a large, dark-red door in Paper Mario: The Thousand-Year Door of a strange architectural design with stars on it; it was said to have a legendary treasure from a thousand years ago inside it. There are many, many myths about what exactly is behind the door, but no one had any idea that it actually led to the dark and eerie Palace of Shadow, the final dungeon in the game.
The door was created to hold back a demon that had destroyed the town that had once been in Rogueport's boundaries. But four heroes made it so that the Crystal Stars were required to open this door. This seal on the door, would only last for a thousand years and was therefore continually weakening before the events of Paper Mario: The Thousand-Year Door.
When Mario placed all seven Crystal Stars before the door, the dark power caused the door and walls around it to crack open. A vortex appeared from the shadows and, for the first time in a thousand years, the Shadow Queen's home was open. Mario then treks through the vast maze within, fighting horrible foes and deadly bosses. The final battle against Grodus and Bowser take place within here.
Throughout the game, many thoughts of the treasure persist of what it might be, but there's never any evidence that supports the citizens' claims. As the ending events of Paper Mario: The Thousand-Year Door transpire, this so-called "treasure" behind the door was eventually thought to be the Shadow Queen herself, but the ending actually showed that there was indeed a treasure of sorts behind it. There was a near-useless Dried Shroom, which could have been a much more powerful mushroom in the past but dried out over time. Its discoverer, Professor Frankly, is still happy with his find because it is proof that civilizations ate mushrooms 1000 years ago.
- A Sammer Guy in Super Paper Mario is called "Thousand-Year Roar", referencing either the Thousand-Year Door itself or Paper Mario: The Thousand-Year Door's subtitle.
- In the final scene in which the player takes control of Bowser, Kammy Koopa immediately recognizes the Thousand-Year Door and its rumored treasure when Bowser asks what it is. It is unknown what the source of her knowledge of the door was; Kammy interestingly seems to be the only non-Rogueport resident who knew of the Thousand-Year Door before the events of Paper Mario: The Thousand-Year Door, though it is possible that she learned of it on her Rogueport-spanning travels with Bowser.